The invention described herein relates to a cylindrical screen designed in particular for the treatment of paper pulp.
In patent WO97/34045 a cylindrical screen for paper pulp was described, in which the filtering elements were tightened between two flanges. The filtering elements were held in position by means of beveled ends, which were fitted into grooves that were also beveled.
The implementation of this arrangement is time-consuming.
In patent EP 0 707 109 a cylindrical screen was described which was composed of overlaying of U-shaped rings, and these rings were secured between two flanges by means of a number of ties. The screen in question was thus prestressed due to the elasticity of the U-shaped rings.
The document DE 196 51 643 describes a filtering screen composed of rods placed in a cage constituted by two flanges connected to each other by bars forming ties. The wires are tightened between the flanges and connected to the bars by support rings that at their external edge are welded to the bars and that at their internal edge are provided with recesses receiving rods having complementary forms. The bars can thus be mounted in the support rings and extracted therefrom when taking away one of the flanges so as to have access to the bars. However, this screen has a fixed support structure in which the support rings are welded onto the bars while respecting the exact alignment of the recesses from one ring to an other. Thus, the mounting of this support structure is relatively complicated and consequently expensive.
An other screen for paper pulp is known from FP 0 893 536. This screen comprises a cage composed by two end flanges and one intermediate support ring that are connected to each other by ties formed by bars extending through these three elements. The bars have threaded ends cooperating with nuts placed on the external surfaces of the flanges. The filtering element is constituted by two metal sheets having openings and which are mounted in circular recesses circular recesses carried out on the internal surfaces of the flanges and on one side and the other of the support ring. Thus, this screen requires a machining of the flanges and also of the intermediate support ring.
The purpose of the present invention is to propose a screen that is more simple to produce and thus less expensive than the screens of prior art. The screen according to the invention is particularly meant to avoid welding between the bars and the support rings as in DE 196 51 643, and to avoid the machining of the intermediate ring and of the flanges as in FP 0 893 536.
The present invention further permits fabrication of screens having filtering elements composed of appropriately dimensioned wires assembled side by side attached to a bearing element instead of the U-shaped rings in patent EP 0 707 109.
The screen in the present invention. is of a type composed of one or several filtering elements held between two flanges by means of a number of bars and the filtering element may be constituted by means of assembly on regularly spaced parallel wire supports, positioned side by side with a slight space betweeen them.
Thus, such a filtering element is axially rigid and positioned upstream from the bars in relation to the direction of flow of the pulp and is held against the bars by the wire supports so as to he held in shape by the bars in order to withstand the pressure exerted by the pulp.
The subject matter of the invention is a cylindrical screen, particularly for paper pulp, of the type comprising at least two flanges connected to each other by means of a number of bars, and at least one thin filtering element rigid in the axial direction, said filtering element being held by support rings and placed upstream from the above mentioned bars, in relation to the direction of flow of the pulp. The filtering element is held against the bars through die support rings that are in contact with the bars, without being attached to them.
In one embodiment, the bars are attached to the flanges by being held against them so that the distance between the two flanges is invariable, which may necessitate the use of thickness adjusting shims to compensate for the errors in the dimensions of the filtering elements.
In an other embodiment, the bars are attached to and held against one flange, and pass through the other, so that the filtering elements can be secured by the second flange.
According to other features of the invention:
the screen furthermore comprises at least one strengthening ring placed between two filtering elements following one after the other so as to receive the bars and fixe them together in order to prevent bending of the same;
the bars are attached to the two flanges and are held against the flanges, abutting in arrangements so that the distance between the two flanges is invariable;
thickness adjustment shims are interpositioned between the filtering element and one or other of the flanges;
the support rings are at an internal or external edge thereof welded to the filtering element, whereas at an other edge, they are in contact with the bar;
a distance of about 25 mm is arranged between two support rings placed one after the other.